Eddy current torque producing device



y 5, 1956 J. G. OETZEL 2,745,974

EDDY CURRENT TORQUE PRODUCING DEVICE Filed March 26, 1952 6 Sheets-Sheet1 C1 2 if. if

nu null In. Q I I l \Qs I I I mo Q ohm George 08 :3 e Z a 0.41.4 P w May15, 1956 J. G. OETZEL EDDY CURRENT TORQUE PRODUCING DEVICE 6Sheets-Sheet 2 Filed March 26, 1952 m, m G LQ? B l May 15, 1956 J. G.OETZEL EDDY CURRENT TORQUE PRODUCING DEVICE 6 Sheets-Sheet 3 Filed March26, 1952 May 15, 1956 J. G. OETZEL 2,745,974

EDDY CURRENT TORQUE PRODUCING DEVICE Filed March 26, 1952 6 Sheets-Sheet4 ll/Iii? Qjohn Gecwgge Oeibel (.HTTORNEY/ May 15, 1956 Filed March 26,1952 ROTATION J. G. OETZEL EDDY CURRENT TORQUE PRODUCING DEVICE 6Sheets-Sheet 5 lll l H imvtrrofi'ok ohv ceoge @82 el y 5, 1956 J. G.OETZEL. 2,745,974

EDDY CURRENT TORQUE PRODUCING DEVICE Filed March 26, 1952 6 Sheets-Sheet6 United States Patent 2,745,974 EDDY CURRENT TORQUE PRODUCING DEVICEJohn George (letzel, Beloit, Wis., assignor to Warner Electric Brake &Clutch Company, South Beloit, Ill., a corporation of IllinoisApplication March 26, 1952, Serial No. 278,662 15 Claims. (Cl. 310-93)This invention relates to eddy current brakes and clutches of the typehaving an inner stator with so-called imbricated or overlapping polesenclosed by an inductor in which eddy currents are induced and fromwhich heat is absorbed by air induced to flow outwardly by the action offan elements rotatable with the inductor.

The general object is to provide an eddy current device which, ascompared with prior devices, is substantially more eflicient, iseffectually cooled with a minimum consumption of driving power, and yetis simple and inexpensive in construction.

Another object is to construct the inductor element of the device in anovel manner to force wider distribution of the eddy currents within themetal of the inductor and thereby reduce the length of the path throughwhich the generated heat must be conducted before reaching the heatdissipating surfaces.

A more detailed object is to divide the inductor into a series ofelongated teeth of tapered cross section each having side surfaces whichconverge radially and outwardly away from the tooth face opposing thepole faces of the stator.

A further object is to provide for expansion of the cooling air as itflows radially between adjacent sides of the inductor teeth whereby toincrease the heat absorbing capacity of the cooling air.

Another object is to arrange the teeth of the inductor and the faces ofthe imbricated poles in a novel angular relationship so as not only tochange the direction of the flux in each inductor tooth rapidly but alsoto shift the flux along the tooth whereby to obtain optimum eddy currentand hysteresis effects.

Still another object is to utilize the inductor teeth themselves as fanblades for inducing the circulation of cooling air.

Other objects and advantages of the invention will become apparent fromthe following detailed description taken in connection with theaccompanying drawings, in which Figure 1 is an elevational view partlyin section of an eddy current device embodying the novel features of thepresent invention.

Fig. 2 is a fragmentary sectional view taken along the line 2-2 of Fig.1.

Fig. 3 is a fragmentary perspective view of the stator and part of theinductor.

Figs. 4 and 5 are fragmentary views similar to Figs. 1 and 2 showing amodification.

Figs. 6 and 7 are views similar to Figs. 4 and 5 showing a modificationof the latter form of the invention.

Fig. 8 is a development of the faces of the stator poles and theinductor teeth.

Figs. 9 and 10 are fragmentary perspective views of the stator poles andinductor teeth showing the progress of the magnetic flux along theteeth.

Fig. 11 is a fragmentary perspective view of the stator of the deviceshown in Fig. 6.

Figs. 12, 13, and 14 are diagrammatic views illustrating variousrelative inclinations of the pole faces and stator teeth.

Fig. 15 shows typical performance curves.

In the forms shown in the drawings, the improved eddy current device isespecially adapted for use as a brake in heavy duty automotive vehiclesfor applying a retarding torque to a rotary part such as a shaft 10projecting from a stationary housing 11 mounted on the vehicle frame 12and supporting the shaft bearings 13. The stator or field member of thebrakes shown in Figs. 1 to 3 comprises two hollow magnet rings 14arranged end to end and having internal flanges 15 bolted together andto the end of the housing 11. Each ring 14 encloses an annular multipleturn coil 16 and its outer periphery flares outwardly and is dividedinto separated or so-called imbricated and axially tapered projections17 and 18 extending from opposite ends of the ring and terminating incircumferentially spaced substantially parallel faces 19 and 20 whichare oppositely polarized substantially to the point of flux saturationwhen the winding 16 is energized and which lie on a cylinder ofrevolution centered on the axis of the shaft 10. The pole projectionscooperate to define air passages through which air entering :between orat the ends of the rings 14 may circulate freely and outwardly betweenthe adjacent pole faces and from the periphery of the stator.

Concentric with and closely encircling the rotor pole faces is anannular rotary inductor formed in accordance with the present inventionby a series of closely spaced elongated teeth 22 overlapping at leasttwo of the pole faces 19 and 20 in all positions of the inductor andconstructed of novel cross section so as to force a wide distribution ofthe flux close to heat radiating surfaces of large area. in the brakeshown in Figs. 1 to 3, the induct-or teeth 22 comprise complete rings ofmagnetic iron only slightly larger in internal diameter than the statorand joined together to form a rigid unit. Each ring is of substantialradial thickness and tapers outwardly from a maximum width at its innercylindrical face 23 to a relatively thin section which extends over asubstantial part of the radius of the ring at the outer edge thereof.While the outward convergence of opposite side surfaces 24 of each ringmay extend over the full radial width, it is preferred as shown toextend the taper over about half of the ring radius leaving the outerhalf portion 25 of substantial uniform Width. Thus, in the form shown,the inner half of the inductor tooth is of substantially triangularsection while the remaining outer half is parallel sided and of athickness substantially less than half that of the inner face 23 of thetooth.

Since each of the teeth 22 encircles the entire periphery of the stator,the successive circumferential lengths of the tooth extend across andoverlap the adjacent pole faces 19 and 20 and thus provide separate lowreluctance paths through which flux may thread from pole to pole, thelongitudinal path between each pair of stator poles shifting around thering as the inductor rotates. As will be described more fully later, thecross sectional area of each inductor tooth and the areas of overlapwith the adjacent pole faces 19 and 29 are correlated in a novelmannerto force a wide distribution of the flux throughout the'crosssection of each tooth so as to facilitate rapid dissipation of heat fromthe tooth surfaces.

In the form shown in Figs. 1 to 3, the ring type inductor teeth 22 arejoined together by a series of closely spaced parallel bars 26 spanningthe outer edge portions of the teeth and constituting fan elements forinducing the flow of a large volume of air outwardly between the poleprojections 17 and the rings 22. The bars are of'uniform thickness andcomparatively thin and disposed in radial planes uniformly spaced aroundthe inductor. Lugs 27 at opposite ends of each bar project into and fitclosely in holes in generally flat rings 28 and 29 to which the bars aresecured rigidly as by copper brazing at all of the adjoining surfaces.

To join the bars 26 and the teeth 22 in good heat conducting relation,the inner edge portion of each bar is notched to receive the outer edgeportion 25 of the teeth and form lugs 29 which fit closely between thesides of the adjacent teeth and are joined thereto as by brazing.Preferably, each lug 29 extends inwardly over substantially the fullwidth of the fiat parts of the inductor teeth. The teeth and the barsare thus joined into a rigid unit which constitutes not only a toothannular inductor with air passages between adjacent teeth but also thecage of a fan which operates efliciently by turning of the inductor toinduce an outward flow of air at a high volumetric rate. Baflle rings 30are preferably provided at opposite ends of the fan cage to minimize theloss of efficiency due to aspiration of external air into the fandischarge at the outer ends of the passages between the adjacent bars26. These rings are welded or brazed to the end rings 28 and 29 andpreferably converge toward each other outwardly from the ends of the fanblades.

It will be observed that owing to outward tapering of the inductor teeth22, the air passages between adjacent teeth increase in Width outwardlyfrom the inlets 33 which comprise relatively narrow slots between theadjacent edges of the teeth at their inner faces 23. Thus, the air drawnoutwardly through the slots 33 expands as it continues outwardly acrossthe adjacent diverging sides 24 of the teeth. Due to such expansion ofthe air, theheat absorption capacity thereof is increased substantially.

The combined inductor and fan unit may as shown he supported from oneend and attached to the shaft 10. For this purpose, the end ring 28constitutes the outer portion of a disk 31 which curves inwardly aroundthe inner periphery of the adjacent magnet 14 and at its inner edge isbolted to a flange 32 on the shaft 10. The inner edge portion of thedisk may be apertured to permit the free entry of air to the interior ofthe adjacent magnet ring 14 whose flange has angularly spaced holes 35formed therein. Similar holes are formed in the mounting flange of theother magnet ring thus permitting the free flow of air outwardly throughand around the rings 14, the pole projections 17 thereof, the inductorteeth 22, and the fan blades 26 as indicated by the arrows in Fig. 1.

In the modified forms shown in Figs. 4 to 11, the construction of theinductor is simplified by utilizing the teeth 22 themselves as the fanelements. To this end, the teeth, which may be of the same cross sectionas the teeth previously described, are disposed in radial planes so asto extend across the periphery of the stator preferably parallel to theaxis thereof. Thus, the teeth take the form of bars of lengths equal tothe axial width of the pole faces 19 and of each stator magnet 14. Theinternal faces 23 are preferably concaved slightly in a circumferentialdirection to conform closely to the curvature of the pole faces 19 and20 so that the air gaps between the two are of uniform minimum thicknessat all points. v

At opposite ends, each tooth 22 abuts against and is copper brazed tothe sides of flat rings 36 and 37 which are apertured to snugly receivelugs 38 formed on the bar ends. The ring 37 comprises the outerperipheral part of a disk 39 similar to the disk 31 previously describedand mounted on the shaft 10. Bafiies 40 (Fig. 6) secured to the rings 36and 37 project outwardly beyond the outer edge portions of the teeth andconverge toward each other for the purpose previously mentioned. Ifdesired in order to reinforce the bars 22 against radial warping, thelatter may be joined intermediate their ends and at several points byrings 41 seated in notches in the outer edge portions of the bars andsecured to the latter by copper brazing. To render the thickened inneredge portions of the bars somewhat flexible and thus reduce the dangerof thermal warping, shallow notches 42 may be formed along these edgesand extended transversely across each bar.

In the form shown in Fig. 4, the end rings 36 and 37 themselves form thebaflles at the ends of the fan outlets and also serve to join the teethtogether at points spaced inwardly from the rings. To this end, therings are formed with flanges 43 turned inwardly toward each other andoverlying opposite end portions of the teeth which slope outwardly asindicated at 45 to notches 44 in which the inner edges of the flanges 43are seated. Preferably, the central part 46 of each tooth projectsoutwardly between the adjacent inner edges of the flanges 43 therebyincreasing the radial width of the fan blades formed by the teeth 22.

It will be observed that the radially disposed and circumferentiallyspaced teeth mounted as described constitute fan blades which, duringrotation of the inductor, induce a high rate of air flow into the narrowslots 33 between the teeth and their inner faces 23. In passingoutwardly, the air expands along the diverging sides 24 of the teeth andfinally is discharged from the outer ends of the passages defined by theflat outer edge portions 25 of the teeth.

As shown in Figs. 4, 6, and 11, the stator of the modified eddy currentdevice is constructed and mounted the same as the stator firstdescribed. To enable each tooth 22 to always provide a low reluctanceflux path between two pole faces of opposite polarity in all positionsof the inductor, the stator pole projections 17 are inclined relative tothe inductor axis at an angle such that the inner face 23 of each toothalways extends across and overlaps one pole face 19 and an adjacent face29 of opposite polarity as indicated by the hatched areas 47 and 48 inFigs. 8, 12, 13, and 14. The desired inclination is achieved simply bymilling out the magnet ring casting to separate the pole projections 17and leave the rectangular pole faces 19 and 26 disposed at the properangle. While the pole faces of the two magnets may be inclined at thesame angle and the corresponding faces alined with each other, it isusually preferable to employ opposite inclinations and arrange the facesherringbone fashion as shown in Figs. 8 and 11.

The particular inclination selected is determined by the widths of polefaces 19 and 2t and the tooth faces 23, the permissible axial length ofthe magnet, and also by the performance characteristics desired with theinductor speed prevailing in a given installation. As shown by Figs. 12and 13 in which the pole faces are inclined at and 30 degreesrespectively, it will be observed that the greater the inclination theshorter may be the axial length of the magnet rings 14. Also, the speedat which the overlapped areas 47 and 48 progress along the tooth willvary inversely with the inclination of the pole faces. If asubstantially straight speed torque curve is desired as indicated at 49(Fig. 15), the stator poles would be inclined at a large angle as shownin Fig. 12. Reducing the angle as shown in Fig. 8 to increase the rateat which the overlapped areas 47 and 48 progress along the pole facesand teeth results in a more rapid increase in torque with speed asindicated at 50 followed by more abrupt flattening of the curve asmaximum speed is attained. A further increase in the slope of thespeed-torque curve as indicated at 51 will result from a still furtherdecrease in the inclination of the pole projections 17 as shown in Fig.13. The speed torque characteristic may also be varied by changing thenumber of stator poles per unit of stator circumference or by changingthe size of the overlapped areas 47 and 48 as by reducing the widths ofthe teeth 22 as shown in Fig. 14. As the number of poles is increased,the angle of inclination may be decreased while retaining a desiredoverlapped area of the pole and tooth faces.

Referring now to Figs. 8 to 10, it will be apparent that each inductortooth 22 provides a low reluctance path to which magnetic flux from onepole face 19 may thread to one end portion of the tooth face 23 and fromwhich the flux may thread back to the area 48 of the adjacent oppositepole face which underlies the opposite end portion of the tooth face.Within the tooth and between the overlapped areas 4.7 and 48, the fluxlines follow a path extending longitudinally of the tooth andprogressing along the latter as indicated in Figs. 9 and 10. As bettershown by the arrows in Fig. 8, the total flux path through any one tooth22 shifts progressively to the right as the tooth advances by rotationof the inductor. So long as the tooth is overlapping two predeterminednorth and soutn poles, the direction of the flux through the toothremains the same as indicated by the arrows 52. When the right end of atooth passes one north pole and the other end starts to overlap the nextnorth pole, the flux path is shifted to the other end poo tion of thetooth and the direction of the flux is reversed as indicated by thearrows 53. Moreover, the density of the flux within the inductor toothchanges as the tooth ends move off from and onto the ends of a statorpole face preparatory to changing the direction of the flux through thetooth. As a result of the rapid cutting of the stator flux by movementof the inductor teeth together with the reversal of the flux, itschanges in density and the direction of its progress along the tooth, asubstantial hysteresis effect is obtained along with the building up ofeddy currents of substantial magnitude within the inductor teeth. Theetficiency of the device in producing torque is thus increasedsubstantially as compared to prior eddy current devices.

By making the end rings 36 and Y37 of magnetic material, these ringsconstitute the seats of additional eddy currents and result in a furtheraugmentation of the torque developed. This is due to the fact thatarcuate portions of these rings form low reluctance magnetic connectionsbetween the ends of the inductor teeth, these connections being threadedby magnetic flux when an overlapped area 47 or 48 is approaching the endof a tooth 22 and is therefore decreasing in size. The heat thusgenerated within the end rings is absorbed by the cooling air flowingoutwardly through the inductor teeth.

By outwardly tapering the cross section of the teeth 22 as describedabove and correlating the section area of the tooth with the areas 47and 48 of overlap between the pole and tooth faces 19, 20, and 23, themagnetic fiux threading the teeth 22 between these areas is forced intoall parts of the tooth section thus causing the heat generated by theeddy currents to be distributed widely within the tooth cross sectionthus facilitating its rapid transfer to the air streams flowingoutwardly between the inductor teeth. In general, the cross section ofeach tooth 2.2 is made substantially smaller than the overlap areas 47and 43 and only sufiicient in size to carry when substantially saturatedall of the flux which threads across the air gaps between the overlappedtooth and pole areas 23, 4-7, and 48. The substantial width of the toothfaces required for this purpose while providing small enough crosssection to approach saturation in the teeth 22 is achieved by convergingthe sides 24 of the teeth outwardly as above described. Widedistribution of the flux to the full depth of the teeth including thethin outer edge portion 25 is achieved by making the inner or triangularpart of the tooth section too small in area to carry all of the fluxentering the tooth from the areas 47 and 48. As a result, some of theflux is forced up into the edge portions 25 and the entire cross sectionof the tooth between the face areas 47 and 48 approaches magneticsaturation.

Since eddy currents tend to follow the surface of the metal part inwhich they are generated and also follow and surround the path of themagnetic flux, most of the heat resulting from the flow of thesecurrents in the teeth 22 will be created near the side surfaces 24 andwill be conducted rapidly to these surfaces for immediate absorption bythe air streams flowing outwardly between the adjacent teeth. Such widedistribution of the developed heat together with the efiicient action ofthe fan in moving air over all of the tooth surfaces results in rapiddisposal of the heat thus enabling the metal to be worked to maximumcapacity with a minimum consumption of cooling air and therefore ofpower for driving the fan. The cooling effect of the air is furtherincreased by virtue of the xpansion of the air as it passes therestricted inlets 33 and passes outwardly between the diverging sides ofthe adjacent teeth. Since the heat generated by eddy current andhysteresis is distributed quite uniformly in the tooth sections, thepossibility of warping of the teeth due to differential heating iseffectually minimized.

If desired, the magnitudes of the eddy currents may be increased anddisposal of the generated heat further facilitated by covering thesurfaces of the teeth with a thin layer indicated at 56 in Fig. 5 ofmetal such as copper of better electrical conductivity than iron. Thismay be achieved for example by electroplating the parts before assemblyor the entire inductor cage after assembly. By this addition the speedtorque curve 51 for example may be raised as indicated at 57 in Fig. 15.

I claim as my invention:

1. A rotary inductor for aneddy current device comprising a series ofcircular rings of magnetic metal and equal size concentric with a commonaxis and disposed side by side in closed spaced relation whereby todefine narrow annular slots between the rings, a series of cross barscircumferentially spaced around and spanning the outer edges of saidrings so as to constitute fan blades operable during rotation of theinductor to induce air to flow radially and outwardly through saidpassage, each of said bars having teeth spaced along its inner edge andeach projecting in between two adjacent rings and connected to the sidesof the latter in heat conducting relation, and means rigidly joiningsaid rings and bars and supporting the same for rotation about saidaxis.

2. A rotary inductor for an eddy current device comprising a series ofcircular rings of magnetic metal and equal size concentric with a commonaxis and disposed side by side in closely spaced relation whereby todefine narrow annular slots between the rings, the opposite sides ofeach ring converging outwardly whereby to define between adjacent ringsan annular passage flaring outwardly from one of said slots, a series ofcross bars spanning the outer edges of said rings and disposed insubstantially radial planes so as to constitute fan blades operableduring rotation of the inductor to induce air to flow radially andoutwardly through said passages, and means rigidly joining said ringsand cross bars and supporting the same for rotation about said axis.

3. An eddy current torque-producing device comprising a field memberhaving an annular series of imbricated pole pieces terminating ingenerally rectangular pole faces facing outwardly and lying on a commoncylinder of revolution, said pole faces being of equal size and equallyspaced and being inclined relative to the axis of said cylinder, meansfor oppositely polarizing the adjacent pole pieces, a series of parallelsubstantially radially disposed teeth of magnetic material extendingacross and angularly spaced around the periphery of said member andhaving a inner generally rectangular faces lying substantially on acylinder concentric with said axis and disposed close to and opposingsaid pole faces, the inclination of said pole faces being such that eachtooth face always overlaps and extends across at least two adjacent polefaces of opposite polarity, and means rigidly joining said rings andsupporting the same for rotation about said axis, the opposite sides ofeach tooth converging outwardly to define between adjacent teethoutwardly flaring air passages.

4. An eddy current torque-producing device comprising a field memberhaving an annular series of imbricated pole pieces terminating ingenerally rectangular pole faces facing outwardly and lying on a commoncylinder of revolution, said pole faces being of equal size and equallyspaced and being inclined relative to the axis of said cylinder, meansfor oppositely polarizing the adjacent pole pieces, an inductor having aseries of parallel rigidly joined teeth of magnetic material extendingacross and angularly spaced around the periphery of said rotor andhaving inner generally rectangular faces lying on a cylinder anddisposed close to and opposing said pole faces, the inclination of saidpole faces being such that each tooth face always overlaps and extendsacross at least two adjacent pole faces of opposite polarity, meansrigidly joining said teeth at opposite ends and supporting the inductorfor rotation about said axis, and means disposed intermediate the endsof said teeth and rigidly joining the latter together.

5. An eddy current torque-producing device comprising a field memberhaving an annular series of imbricated pole pieces terminating ingenerally rectangular pole faces facing outwardly and lying on a commoncylinder of revolution, said pole faces being of equal size and equallyspaced and being inclined relative to the axis of said cylinder, meansfor oppositely polarizing the adjacent pole pieces, an inductor having aseries of parallel rigidly joined teeth of magnetic material extendingacross and angularly spaced around the periphery of said rotor andhaving inner generally rectangular faces lying on a cylinder anddisposed close to and opposing said pole faces, the inclination of saidpole faces being such that each tooth face always overlaps and extendsacross at least two adjacent pole faces of opposite polarity, and meansrigidly joining said teeth at opposite ends and supporting the inductorfor rotation about said axis.

6. An eddy current torque-producing device comprising a field memberhaving an annular series of imbricated pole pieces terminating ingenerally rectangular pole faces facing outwardly and lying on a commoncylinder of revolution, said pole faces being of equal size and equallyspaced and being inclined relative to the axis of said cylinder, meansfor oppositely polarizing the adjacent pole pieces, an inductor having aseries of parallel rigidly joined teeth of magnetic material extendingacross and angularly spaced around the periphery of said rotor andhaving inner generally rectangular faces lying on a cylinder anddisposed close to and opposing said pole faces, the inclination of saidpole faces being such that each tooth face always overlaps and extendsacross at least two adjacent pole faces of opposite polarity, and meansrigidly joining said teeth at opposite ends and supporting the inductorfor rotation about said axis, each of said teeth being notched at pointsspaced along the inner face thereof.

7. An eddy current torque-producing device comprising a field memberhaving an annular series of imbricated pole pieces terminating ingenerally rectangular pole faces facing outwardly and lyingsubstantially on a common cylinder of revolution, said pole faces beingof substantially equal size and spacing and being inclined relative tothe axis of said cylinder, means for oppositely polarizing the adjacentpole pieces, a rotary inductor comprising a series of parallel teeth ofmagnetic material having inner generally rectangular faces lying on acylinder closely encircling said pole faces, and means rigidly joiningsaid teeth and supporting the latter to turn about said axis, said toothfaces being inclined relative to said pole faces so that each tooth faceoverlaps and extends across at least two adjacent pole faces of oppositepolarity whereby the flux path through the tooth between the overlappedareas of the pole faces shifts longitudinally of the tooth duringrelative rotation between said field member and said inductor, and theopposite sides of each of said teeth converging radially and outwardly.

8. A rotary inductor element foran eddy current torque-producing devicecomprising a series of elongated teeth disposed side by side in closelyspaced relation with their internal faces lying substantially on acylinder of revolution and defining narrow inlet passages at theiradjacent edges, and means rigidly connecting said teeth and supportingthe same for rotation about the axis of said cylinder, the oppositesides of each tooth converging outwardly whereby the adjacent sidesurfaces of adjacent teeth cooperate to define air passages flaringoutwardly from said inlets.

9. A rotary inductor element for an eddy current torque-producing devicecomprising a series of elongated teeth disposed side by side in closelyspaced relation with their internal faces lying substantially on acylinder of revolution and defining narrow inlet passages at theiradjacent edges, and means rigidly connecting said teeth and supportingthe same for rotation about the axis of said cylinder, the oppositesides of each tooth converging outwardly to a parallel side rib alongthe outer edge of the tooth whereby the adjacent side surfaces ofadjacent teeth define air passages flaring outwardly from said inlets.

10. An eddy current torque-producing device com prising a field memberhaving an annular series of imbricated pole pieces terminating ingenerally rectangular pole faces facing outwardly and lyingsubstantially on a common cylinder of revolution means for oppositelypolarizing the adjacent pole pieces, a rotary inductor comprising aseries of parallel teeth of magnetic material arranged side by side andforming an annulus closely encircling said field member with the face atthe inner edge of each tooth inclined relative to said pole faces sothat each tooth face in each position of the inductor overlaps andextends across at least two pole faces of opposite polarity, .theopposite side surfaces of each tooth converging radially and outwardlyto taper the cross section of the tooth, and means rigidly joining saidteeth and supporting the same for rotation about said axis, the Widthsof said pole and tooth faces being correlated with the area of saidtooth cross section whereby the maximum area of each overlap betweensaid tooth face and pole faces is substantially greater than said crosssection area whereby to force magnetic flux into the outer edge portionof the tooth section.

11. An eddy current torque-producing device comprising a field memberhaving an annular series of imbricated pole pieces terminating ingenerally rectangular pole faces facing outwardly and lyingsubstantially on a common cylinder of revolution, said pole faces beingelongated in the direction of and inclined relative to the axis of saidcylinder, means for oppositely polarizing the adjacent pole pieces, arotary inductor comprising a series of parallel teeth of magneticmaterial arranged side by side and forming an annulus closely encirclingsaid field member with the face at the inner edge of each toothsubstantially paralleling said axis at an angle such that each toothface always overlaps and extends across at least two pole faces ofopposite polarity, the opposite side surfaces of each tooth convergingradially and outwardly to taper the cross section of the tooth, andmeans rigidly joining said teeth and supporting the same for rotationabout said axis, the widths of said pole and tooth faces beingcorrelated with the area of said tooth cross section whereby the area ofoverlap between said tooth face and pole faces is substantially greaterthan said cross section.

12. An eddy current torque-producing device comprising a field memberhaving an annular series of imbricated pole pieces terminating in polefaces facing outwardly and lying substantially on a common cylinder ofrevolution, means for oppositely polarizing the adjacent pole pieces arotary inductor comprising a series of parallel teeth of magneticmaterial arranged side by side and forming an annulus closely encirclingsaid field member with the face at the inner edge of each tooth beinginclined relative to said pole faces so that each tooth face in eachposition of the inductor overlaps and extends across at least two polefaces of opposite polarity, the opposite side surfaces of each toothconverging radially and outwardly to taper the cross section of thetooth, and means rigidly joining said teeth and supporting the same forrotation about said axis, the maximum area of each overlap between saidtooth face and pole faces being sufiiciently greater than said crosssection area to effect substantial flux saturation of said cross sectionbetween the overlapping areas of the tooth and pole faces.

13. A rotary inductor for an eddy current torque-producing devicecomprising a series of elongated teeth of magnetic material disposedside by side in closely spaced relation with their internal faces lyingsubstantially on a cylinder of revolution and defining air inlets, andmeans rigidly connecting said teeth and supporting the same for rotationabout the axis of said cylinder, said teeth extending longitudinally ofthe axis of said cylinder whereby the sides of said teeth act duringrotation of the inductor as fan elements inducing the flow of airoutwardly through each of said inlets and across the teeth.

14. A rotary inductor for an eddy current device comprising axiallyspaced rings of magnetic material, and a series of teeth of magneticmaterial each spanning and 25 2538797 rigidly connecting said rings andangularly spaced around 10 the latter whereby to form a fan operableduring rotation of the inductor to induce an outward flow of air betweenthe teeth, the inner surfaces of said teeth lying substantially on acommon cylinder of revolution and the cross section of each toothdecreasing in width outwardly from said inner faces.

15. A rotary inductor for an eddy current device comprising axiallyspaced rings of magnetic material, a series of teeth of magneticmaterial each spanning and rigidly connecting said rings and angularlyspaced around the latter whereby to form a fan operable during rotationof the inductor to induce an outward flow of air between the teeth, theinner surfaces of said teeth lying substantially on a common cylinder ofrevolution and the cross section of each tooth decreasing in widthoutwardly from said inner faces, and a layer of metal of high electricalconductivity covering said inner faces and the sides of said teeth.

References Cited in the file of this patent UNITED STATES PATENTS853,283 Waters May 14, 1907 2,401,187 Prince May 28, 1946 Oetzel Ian.23, 1951

